[LLVMbugs] [Bug 16626] New: Objective-C method writeback parameter marked "out" does not allow passing more generic pointer type, unless -fobjc-arc is enabled

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=16626

            Bug ID: 16626
           Summary: Objective-C method writeback parameter marked "out"
                    does not allow passing more generic pointer type,
                    unless -fobjc-arc is enabled
           Product: clang
           Version: 3.3
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jtbandes at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

With the following classes:

    @interface Foo @end
    @implementation Foo @end

    @interface Bar : Foo @end
    @implementation Bar @end

    @interface Nothing
    + (void)blah:(out Bar **)b;
    @end
    @implementation Nothing
    + (void)blah:(out Bar **)b { }
    @end

One would expect to be able to pass the address of a Foo* object in for the
writeback parameter, since the +blah: method is expected to write an object of
type Bar*, a subtype of Foo*. For example:

    int main (int argc, char const *argv[])
    {
      Foo *f;
      [Nothing blah:&f];

      return 0;
    }

This works when -fobjc-arc is enabled, but otherwise, the following warning is
given:

    warning: incompatible pointer types sending 'Foo **' to parameter of type
'Bar **' [-Wincompatible-pointer-types]

Note: the "in"/"out"/"inout" annotations have historically been part of
Distributed Objects, but still have meaning nowadays, as seen in Clang's ARC
documentation:

http://clang.llvm.org/docs/AutomaticReferenceCounting.html#passing-to-an-out-parameter-by-writeback

"A pass-by-writeback is evaluated as follows:
...
4. If the parameter is not an Objective-C method parameter marked out, then *p
is read, and the result is written into the temporary with primitive
semantics."

In my opinion these annotations/qualifiers should be enough for the compiler to
realize, in all cases, that passing a Foo** value to an "out Bar**" parameter
is completely valid. (This issue was inspired by the minor outrage against the
new Dropbox Obj-C API, which provides DBError, a subclass of NSError, and has
some DBError** writeback parameters. Dropbox did not mark these parameters
"out", but if they had, it should have not caused any problems to pass an
NSError**.)

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